Mass Wasting and Landslides

Mass Wasting and Landslides Mass Wasting 1 Introduction Landslide and other ground failures posting substantial damage and loss of life In U.S., average 25–50 deaths; damage more than $3.5 billion For convenience, definition of landslide includes all forms of mass-wasting movements Landslide and subsidence: naturally occurred and affected by human activities Mass wasting Downslope movement of rock and soil debris under the influence of gravity Transportation of large masses of rock Very important kind of erosion 2 Mass wasting Gravity is the driving force of all mass wasting Effects of gravity on a rock lying on a hillslope 3 Boulder on a hillside Mass Movement Mass movements occur when the force of gravity exceeds the strength of the slope material Such an occurrence can be precipitated by slope-weakening events Earthquakes Floods Volcanic Activity Storms/Torrential rain Overloading the strength of the rock 4 Mass Movement Can be either slow (creep) or fast (landslides, debris flows, etc.) As terrain becomes more mountainous, the hazard increases In developed nations impacts of mass-wasting or landslides can result in millions of dollars of damage with some deaths In less developed nations damage is more extensive because of population density, lack of stringent zoning laws, scarcity of information and inadequate preparedness **We can’t always predict or prevent the occurrence of mass- wasting events, a knowledge of the processes and their relationship to local geology can lead to intelligent planning that will help reduce losses of life and property Controls and Triggers of Mass Wasting Important factors include: The role of water Diminishes particle cohesion (friction) Water adds weight Oversteepening of slopes—slope angle Stable slope angle (angle of repose) is different for various materials Oversteepened slopes are unstable 5 Angle of Repose- Effect of Grain Size Angle of Repose- Effect of Moisture Content 6 How do they do that? Surface Tension 7 Unsaturated Soil Saturated Soil Slope Instability Caused by Oversteepening 8 Water causing failure in slopes of solid bedrock Water causing failure in slopes of solid bedrock 9 Angle of Repose S. C. Porter Controls and Triggers of Mass Wasting Important factors Removal of anchoring vegetation Root system binds soil and promotes stability Earthquakes as triggers May cause expensive property damage Can cause liquefaction—Water-saturated surface materials behave as fluid-like masses that flow 10 Fire increases the susceptibility for erosion and mass movement Controls and Triggers of Mass Wasting Landslides without triggers Slope materials weaken over time. Random events that are unpredictable 11 Weathered Shale Fig. 11.6 Martin Miller Classification of Mass Wasting Processes Generally, each event is classified by: Type of material involved Mud, Earth, Rock Type of motion Fall (free-falling pieces) Slide (material moves along a surface as a coherent mass) Flow (material moves as a chaotic mixture) The velocity of the movement Fast or Slow 12 Mass Wasting Processes Rockfalls 13 Sunday May 9, 1999 50 people injured 8 people dead Rockfall was small -25-30 cubic yards -Weathered basalt -500 feet above 14 15 16 Bad Car Day 17 Slump 18 What causes slumps? •Most common reason for slumping • Erosion at the base of the slope • Reduces the support for overlying sediments •Erosion at the base of a slope has many causes • Stream channel, wave action, seepage of water into the ground during the rainy season, etc. Wave cut wall Processes that oversteepen slopes 19 Processes that oversteepen slopes Processes that oversteepen slopes 20 Slopes susceptible to mass movement 21 Debris flows Occur when the rock/soil mass loses coherency and lots of water is involved Debris becomes mixed up completely and flows as liquid mud Often carry large clasts Can be very destructive 22 Debris flow 23 24 Debris avalanche 25 Debris slide 26 Earthflow 27 Mudflow 28 Soil Creep Creep Gradual movement of soil and regolith downhill Aided by the alternate expansion and contraction of the surface material Mass movement that moves very slowly Weak soils on steep slopes move slowly down hill 29 Some Visible Effects of Creep 30 Landslides Occur when a large piece of rock and/or soil breaks off and slides down hill Often initiated by earthquakes and by very heavy rainstorms Can occur when humans foolishly overload a road bordering a canyon or steep drop off with heavy equipment Landslides Initiated when rock/soil originally held in place by internal cohesion suddenly loses that cohesion Form on slopes that are steep enough for the weight of the surficial material to overcome the cohesive force and fail 31 Steep Headwall Spoon-shaped sliding surface 32 Jumbled-up Toes 33 Landslides Tendency to slide increases with increasing surface slope Addition of water promotes sliding by adding weight and by reducing cohesion Friction along the slide surface controls the speed of the downslope movement Landslides Addition of water reduces friction along the surface and allows the mass to slide faster Some slides move as coherent mass Others break up and the material inside becomes jumbled and disorganized 34 If bedding dips down-slope, a landslide is more likely to develop 35 Landslides Certain areas are more prone than others Usually because of local geological factors Reducing vegetation that was stabilizing the soil Over watering (decrease friction) 36 Dangerous OK to cut to cut Cutting a slope during construction can cause a landslide 37 LaConchita (S. California) landslide, January, 1995 38 LaConchita (S. California) landslide, January, 2005 LaConchita (S. California) landslide, January, 2005 39 Our Islands Are Unstable From Shoreline to Mountains Even the youngest parts of the volcanoes are subject to landsliding 40 41 Dating mass-movement events 42 Landslide Induced by Poor Construction Techniques 43 Human Land Use and Landslide Urbanization, irrigation Timber harvesting in weak, relatively unstable areas Artificial fillings of loose materials Artificial modification of landscape Dam construction 44 Minimizing the Landslide Hazard Identifying potential landslides ¾ Photographic analysis ¾ Topographic map and detailed field check ¾ Historic data Landslide hazard inventory map ¾ Grading code from the least stable to the most stable Application of geologic and engineering knowledge before any hillside development Figure 9.16 45 46 Minimizing the Landslide Hazard Preventing landslides ¾ Drainage control: Reducing infiltration and surface runoff ¾ Slope grading: Reducing the overall slope ¾ Slope supports: Retaining walls or deep supporting piles Avoid landslide hazards ¾ Landslide warning for critical evacuations ¾ Correcting landslides 47 48 49 Warning of Impending Landslides Monitoring changes ¾ Human surveillance ¾ Instrumental survey: Tilt meter and geophones Landslide warning system ¾ Info for public awareness and education ¾ Enough time for public evacuation ¾ Stop or reroute traffic flow ¾ Emergency services 50 What Can You Do? Professional geologic evaluation for a property on a slope Avoid building at the mouth of a canyon, regardless of its size Consult local agencies for historical records Watch signs of little slides—often precursor for larger ones What Can You Do? Look for signs of structure cracks or damage prior to purchase Be wary of pool leaking, tilt of trees and utility poles Look for linear cracks, subsurface water movement Engineering solutions (e.g., retaining walls: to stabilize slopes, drainage pipes: to drain water, grade slopes: to reduce oversteepening and diversion walls: to protect structures) Put observations into perspective, one aspect may not tell the whole story 51.

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